The inclusions constituting impurities in steels can be of the following composition: oxides, sulfides, phosphides, silicates, aluminates, nitrides, arsenides, etc. or composites of the same compounds, perhaps complex compounds thereof. The inclusions themselves can be exogenous or endogenous. It is well known that the development of endogenous inclusions is initiated by adding an inclusion-removing alloy or by change of solubility.
At the appropriate temperature, primary inclusions can be relatively easily removed from the steel bath by the addition of an inclusion-removing alloy. When the proper inclusion-removing alloy is used, the removal can be almost complete.
Those alloys are suitable for the purpose, which produce insoluble inclusions of lower specific gravity and lower melting point than these of the steel. The processes applied should promote the floating of the inclusion in the metal bath.
Upon casting following the removal of primary inclusions, the metal melt cools down and secondary inclusions appear due to the change of the equilibrium constant. The removal of these secondary inclusions is more complicated than the removal of the primary inclusions and their total removal is practically impossible.
Between the liquidus and solidus lines (i.e. in the liquid+solid two-phase range), it is not possible to remove the tertiary inclusions stuck along the grain boundries due to the segmentation of the inclusions. Furthermore, it is not possible to remove quaternary inclusions segregating at locations energetically above the average places (pores, grain boundaries, dislocations) during the polymorphous transformation due to the reduced solubility. These inclusions remain in the metal at room temperature.
The greatest part of inclusions in a steel are the most injurious oxide inclusions. Their removal or reduction is therefore of great significance. So we deal with these inclusions first. At the same time, however, it should be emphasized that the process may be applied for removing other inclusions as well.
The amount of oxide inclusions in a steel at room temperature depends on the oxygen activity level which can be influenced by deoxidation.
The deoxidation is a very complicated and complex metallurgical process and is influenced by many factors, e.g. deoxidation capability, quantity composition, melting point, extent and speed of solubility, etc. of the deoxidation element. Furthermore, the temperature and oxidation degree of the bath, the amount of other additives, physical and chemical characteristics, growth and removal of the deoxidation products, also play important roles. Among these factors the deoxidation capability of the deoxidant is of major importance from the point of view of the effectivity of deoxidation.
Altough the deoxidation is rather complicated as a metallurgical process, it is carried out even nowadays by simply throwing the deoxidant onto the surface of the metal bath. Only recently have blasting lances and inert gas streams for leading the deoxidant into the metal melt been used.
In special cases the deoxidation is carried out in vacuum in order to avoid the reaction of the deoxidation material and the oxygen of the air.
The Hungarian Pat. No. 174,104 deals with the removal of the primary endogenous inclusions segregating under the influence of the inclusion-removing alloy. Several methods for removing the inclusions from the bath as well as the composition of an inclusion-removing alloy are disclosed.
This inclusion-removing alloy most suitable for removing the inclusions from steels contains 40-50% silicon, 15-30% aluminum, 10-25% calcium, 1.5-15% manganese as well as 2-20% titanium, zirconium, niobium, hafnium, cerium, boron and the rest iron.
The above solution is, however, suitable only for removing the primary inclusions and may not be applied to reduce the quantity of secondary inclusions or to refine the steel structure.